DSpace at VNU: Study of psi(2S) production and cold nuclear matter effects in pPb collisions at root s(NN)=5 TeV

DSpace at VNU: Study of psi(2S) production and cold nuclear matter effects in pPb collisions at root s(NN)=5 TeV tài liệ...

Published for SISSA by Springer

Received: January 29, 2016 Accepted: February 29, 2016 Published: March 18, 2016

Study of ψ(2S) production and cold nuclear matter

The LHCb collaboration

Abstract: The production of ψ(2S) mesons is studied in dimuon final states using

proton-lead (pPb) collision data collected by the LHCb detector The data sample corresponds

transverse momentum less than 14 GeV/c and rapidity y in the ranges 1.5 < y < 4.0 and

−5.0 < y < −2.5 in the nucleon-nucleon centre-of-mass system The forward-backward

production ratio and the nuclear modification factor are determined for ψ(2S) mesons

Using the production cross-section results of ψ(2S) and J/ψ mesons from b-hadron decays,

the b¯b cross-section in pPb collisions at √sN N = 5 TeV is obtained

physics, Heavy Ion Experiments, Heavy-ion collision

Contents

The quark-gluon plasma (QGP) is a state of matter with asymptotically free partons,

which is expected to exist at extremely high temperature and density It is predicted that

heavy quarkonium production will be significantly suppressed in ultrarelativistic

important signatures for the formation of the QGP Heavy quarkonium production can

also be suppressed in proton-nucleus (pA) collisions, where hot nuclear matter, i.e QGP,

is not expected to be created and only cold nuclear matter (CNM) effects exist Such CNM

effects include: initial-state nuclear effects on the parton densities (shadowing); coherent

energy loss consisting of initial-state parton energy loss and final-state energy loss; and

CNM, and to provide essential input to the understanding of nucleus-nucleus collisions

Nuclear effects are usually characterized by the nuclear modification factor, defined as

the production cross-section of a given particle per nucleon in pA collisions divided by that

proton-nucleon system Throughout this paper, y always indicates the rapidity in the proton-

nucleon-nucleon centre-of-mass system

The suppression of quarkonium and light hadrons at large rapidity has been observed

collisions recorded at the LHC in 2013 enable the study of CNM effects at the TeV scale

With these pPb data, the production cross-sections of prompt J/ψ mesons, J/ψ mesons

from b-hadron decays, and Υ mesons were measured, and the CNM effects were studied

“back-ward” directions are defined with respect to the direction of the proton beam The ratio

The advantage of measuring this ratio is that it does not rely on knowledge of the

pro-duction cross-section in pp collisions Furthermore, part of the experimental systematic

uncertainties and theoretical scale uncertainties cancel in the ratio

at central rapidity for ψ(2S) mesons than for J/ψ mesons, while at forward rapidity the

suppressions were compatible within large uncertainties The PHENIX experiment made

experi-ment measured the ψ(2S) suppression in the forward and backward rapidity regions in pPb

J/ψ and ψ(2S) mesons, and so cannot explain the observations One explanation for the

fixed-target results is that the charmonium states produced at central rapidity spend more

time in the medium than those at forward rapidities; therefore the loosely bound ψ(2S)

picture it is expected that the charmonium states will spend a much shorter time in the

CNM at LHC energies than at lower energies, leading to similar suppression for ψ(2S) and

J/ψ mesons even at central rapidity

The excellent reconstruction resolution of the LHCb detector for primary and

produced directly from pp collisions, from those originating from b-hadron decays (called

“ψ(2S) from b” in the following) In this analysis, the production cross-sections of prompt

the production sections of ψ(2S) from b and J/ψ from b, the bb production

cross-section in pPb collisions is obtained

2 Detector and datasets

pseudorapidity range 2 < η < 5, designed for the study of particles containing b or c

quarks The detector includes a high-precision tracking system consisting of a silicon-strip

vertex detector surrounding the pPb interaction region, a large-area silicon-strip detector

located upstream of a dipole magnet with a bending power of about 4 Tm, and three

sta-tions of silicon-strip detectors and straw drift tubes placed downstream of the magnet

The tracking system provides a measurement of momentum, p, of charged particles with

a relative uncertainty that varies from 0.5% at low momentum to 1.0% at 200 GeV/c The

minimum distance of a track to a primary vertex, the impact parameter, is measured with

to the beam, in GeV/c Different types of charged hadrons are distinguished using

infor-mation from two ring-imaging Cherenkov detectors Photons, electrons and hadrons are

identified by a calorimeter system consisting of scintillating-pad and preshower detectors,

an electromagnetic calorimeter and a hadronic calorimeter Muons are identified by a

system composed of alternating layers of iron and multiwire proportional chambers The

online event selection is performed by a trigger, which consists of a hardware stage, based

on information from the calorimeter and muon systems, followed by a software stage, which

applies a full event reconstruction

With the proton beam travelling in the direction from the vertex detector to the muon

system and the lead beam circulating in the opposite direction, the LHCb spectrometer

covers forward rapidities With reversed beam directions backward rapidities are accessible

The data sample used in this analysis is collected from the pPb collisions in early 2013,

magnitude below the nominal LHCb luminosity for pp collisions Therefore, the data were

taken using a hardware trigger which simply rejected empty events The software trigger

for this analysis required one well-reconstructed track with hits in the muon system and

acceptance and reconstruction efficiencies The simulation samples are reweighted so that

the track multiplicity distribution reproduces the experimental data of pPb collisions at

√

generated particles with the detector, and its response, are implemented using the Geant4

toolkit [31,32] as described in ref [33]

3 Event selection and cross-section determination

The ψ(2S) candidates are reconstructed using dimuon final states from events with at least

one primary vertex The tracks should be of good quality, have opposite sign charges and

a common vertex with good vertex fit quality, and the reconstructed ψ(2S) mass should

Due to the small size of the data sample, only one-dimensional differential cross-sections

are measured The differential production cross-section of ψ(2S) mesons in a given

kine-matic bin is defined as

dσ

N

reconstructed with the dimuon final state in the given bin of X, ∆X is the bin width,

The integrated luminosity of the data sample used in this analysis was determined

using a van der Meer scan, and calibrated separately for the pPb forward and backward

samples [37] The kinematic region of the measurement is pT < 14 GeV/c and 1.5 < y < 4.0

(−5.0 < y < −2.5) for the forward (backward) sample For the single differential

bins with edges at (0, 2, 3, 5, 7, 14) GeV/c The rapidity range is divided into five bins

of width ∆y = 0.5

4 Signal extraction and efficiencies

The numbers of prompt ψ(2S) and ψ(2S) from b in each kinematic bin are determined

from an extended unbinned maximum likelihood fit performed simultaneously to the

defined as

tz = (zψ− zPV) × Mψ

The invariant mass distribution of the signal in each bin is modelled by a Crystal Ball

and the other parameters are allowed to vary For differential cross-section measurements,

the sample size in each bin is very small Therefore, in order to stabilise the fit, the mass

resolution of the CB function is fixed to the value obtained from the J/ψ sample, scaled

the combinatorial background is described by an exponential function with variable slope

for prompt ψ(2S) and an exponential function for the component of ψ(2S) from b, both

= 5 TeV

NN s

1 10

pPb(Bwd)

LHCb

2.5

− <

Figure 1 Projections of the fit results to (top) the dimuon invariant mass Mµµ and (bottom)

the pseudo proper decay time t z in (left) pPb forward and (right) backward data In all plots the

total fitted function is shown by the (black) solid line, the combinatorial background component is

shown as the (green) hatched area, the prompt signal component by the (blue) shaded area, and

the b-component by the (red) light solid line.

convolved with a Gaussian resolution function The width of the resolution function and

in each kinematic bin is modelled with an empirical function determined from sidebands

of the invariant mass distribution

backward samples The combinatorial background in the backward region is higher than

that in the forward region, because the track multiplicity in the backward region is larger

estimated signal yield for prompt ψ(2S) mesons in the forward (backward) sample is 285 ±

34 (81 ± 23), and that for ψ(2S) from b in the forward (backward) sample is 108 ± 16

(21 ± 8), where the uncertainties are statistical only

and tz as discriminating variables [40] The total efficiency εi, which depends on pTand y,

includes the geometrical acceptance, the reconstruction efficiency, the muon identification

efficiency, and the trigger efficiency The acceptance and reconstruction efficiencies are

determined from simulation, assuming that the produced ψ(2S) mesons are unpolarised

The efficiency of the muon identification and the trigger efficiency are obtained from data

using a tag-and-probe method as described below

prompt from b inclusive prompt from b inclusive Correlated between bins

Several sources of systematic uncertainties affecting the production cross-section

The uncertainty on the muon track reconstruction efficiency is studied with a

data-driven tag-and-probe method, using a J/ψ sample in which one muon track is fully

Taking into account the difference of the track multiplicity distribution between data and

simulation, the total uncertainty is found to be 1.5%

The uncertainty due to the muon identification efficiency is assigned to be 1.3% for both

It is estimated using J/ψ candidates reconstructed with one muon identified by the muon

system and the other identified by selecting a track depositing the energy of a

minimum-ionising particle in the calorimeters

The trigger efficiency is determined from data using a sample unbiased with respect

to the trigger decision The corresponding uncertainty of 1.9% is taken as the systematic

uncertainty due to the trigger efficiency

To estimate the uncertainty due to reweighting the track multiplicity in simulation,

calculated with these two efficiencies is considered as the systematic uncertainty, which

is less than 0.7% in the forward sample, and about 1.7% in the backward sample

and simulation can introduce a systematic uncertainty To estimate the size of this effect

the acceptance and reconstruction efficiencies have been checked by doubling the number of

uncertainty, which is 0.2% − 10% (0.7% − 23%) in the forward (backward) sample For the

backward sample the separation into prompt ψ(2S) and ψ(2S) from b was not done in bins

Table 2 Integrated production cross-sections for prompt ψ(2S), ψ(2S) from b, and inclusive

ψ(2S) in the forward region and the backward region The p T range is p T < 14 GeV/c The first

uncertainty is statistical and the second is systematic.

The luminosity is determined with an uncertainty of 1.9% (2.1%) for the pPb forward

The combined uncertainty related to the track quality, the vertex finding and the radiative

tail is estimated to be 1.5%

The uncertainty due to modelling the invariant mass distribution is estimated by using

the signal shape from simulation convolved with a Gaussian function, or by replacing the

exponential function by a second-order polynomial The maximum differences from the

nominal results are taken as the systematic uncertainties due to the mass fit To estimate

the corresponding systematic uncertainty on the differential production cross-section due

to the fixed mass resolution, the mass resolution is shifted by one standard deviation

distribution is estimated by fitting the signal sample extracted from the sPlot technique

using the invariant mass alone as the discriminating variable

6 Results

The differential cross-sections of prompt ψ(2S), ψ(2S) from b and inclusive ψ(2S) in the

backward data sample, no attempt is made to separate prompt ψ(2S) and ψ(2S) from b

due to the small statistics However, these two components are separated for the integrated

production cross-sections All these cross-sections decrease with increasing |y|

The integrated production cross-sections for prompt ψ(2S), ψ(2S) from b, and their

region, 2.5 < |y| < 4.0, are also given in the table

The production cross-sections, σ(bb), of the bb pair can be obtained from

(2 ψ Prompt

b

) from

S

(2 ψ

)

S

(2 ψ Inclusive )

S

(2 ψ Prompt

b

) from

S

(2 ψ

= 5 TeV

NN

s

pPb(Fwd) LHCb

c

< 14 GeV/

T

p

Figure 2 Differential cross-section of ψ(2S) meson production as a function of (left) pT and

(right) y in pPb forward collisions The (black) dots represent inclusive ψ(2S), the (blue) triangles

indicate prompt ψ(2S), and the (red) squares show ψ(2S) from b The error bars indicate the total

)

S

(2 ψ Inclusive = 5 TeV

NN

s

pPb(Bwd) LHCb

c

< 14 GeV/

T

p

Figure 3 Differential cross-section of ψ(2S) meson production as a function of (left) pTand (right)

y in pPb backward collisions The error bars indicate the total uncertainties.

cross-sections obtained from ψ(2S) from b are consistent with those from J/ψ from b

is taken into account The systematic uncertainties due to the muon identification, the

tracking efficiency, and the track quality are considered to be fully correlated The

system-atic uncertainties due to the luminosities are partially correlated The averaged results are

also shown in table3

Cold nuclear matter effects on ψ(2S) mesons can be studied with the production

Table 3 Production cross-sections σ(bb) of bb pairs in pPb collisions obtained from the production

cross-sections of J/ψ and ψ(2S) from b The superscript ψ denotes J/ψ or ψ(2S) The first

uncertainties are statistical, the second are systematic, and the third are due to the production

branching fractions The last row gives the average of the J/ψ and ψ(2S) results taking account of

their correlation The correlated and uncorrelated uncertainties are provided separately.

range (2.5 < |y| < 4.0) The results are

taking into account minimum and maximum nuclear shadowing effects, with many of them

predictions of a coherent parton energy loss effect both in initial and final states, with

or without additional parton shadowing effects according to EPS09 The single free

parameterisation is (not) taken into account Within uncertainties the measurements agree

with all these calculations

pp collisions at 5 TeV is needed, which is not yet available However, it is reasonable to

systematic uncertainty due to this assumption is taken to be negligible compared with the

statistical uncertainties in this analysis The ratio R of nuclear matter effects between